Multi-user, High Repetition-Rate,
Soft X-ray FEL User Facility
(based on a Collinear Dielectric Wakefield Accelerator)
John Power, Argonne
Euclid Techlabs LLC:
Argonne National Laboratory, HEP:
Argonne National Laboratory, APS:
Northern Illinois University:
C.Jing, A.Kanareykin, P.Schoessow
W.Gai, G.Ha, C.Li, J.G.Power
R.Lindberg, A.Zholents
P.Piot
Assessment of Opportunities
High Brightness Beams Workshop, San Juan, Puerto Rico, March 25, 2013
Multi-user, High Rep Rate, Soft X-ray FEL User Facility
Low-emittance injector:
• 1 MHz bunch rep. rate
50 MeV
Flexible x-ray beamlines
Lasers linked with a
fiber-optics time
distribution network
•
•
•
•
Tunable pulse length
Seeded
2 color seeded
SASE
2 GeV
experimental end stations
Capable of serving
~2000 scientists/year
Beam spreader
• 100 kHz bunch rep. rate
2
Multi-user soft x-ray FEL facility based on: SRF linac
Capable of serving
~2000 scientists/year
Low-emittance injector:
• 1 MHz bunch rep. rate
Flexible x-ray beamlines
Lasers linked with a
fiber-optics time
distribution network
•
•
•
•
Tunable pulse length
Seeded
2 color seeded
SASE
50 MeV
~ 50 m
experimental end stations
750m
2 GeV
~ 300 m
~ 250 m
~100 m
CW superconducting linac
~1MHz bunch rep. rate
~2 GeV beam energy
~1 kA peak current
~50 m
Beam spreader
• 100 kHz bunch rep. rate
3
Multi-user soft x-ray FEL facility based on: DWFA linac
Dielectric Wakefield Acceleration (DWFA) linac
750m
~50 m
2 GeV
experimental end stations
350m
200 MeV
~50 m
Facility Footprint
350m x 250m
~25 m ~50 m ~30 m
Compact
Beam
Spreader
~100 m
~50 m
Compact DWFA linac
Beam
Shaper
~1MHz bunch rep. rate
~2 GeV beam energy
~1 kA peak current
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Ultra-flexible facility
Dielectric Wakefield Acceleration (DWFA) linac
Flexible
accelerator
beamlines
Flexible
x-ray
beamlines
1.2 GeV
100 pC
0.5 keV
X-rays
2.4 GeV
50 pC
1 keV
X-rays
…
…
Configurable
DWFA Accelerator
End Stations
…
…
Configurable
FEL Array
5
Motivation for DWFA for the High Rep Facility






Low energy spreader
Accelerating gradient > 100 MV/m
Room temperature quartz fibers
Tunable electron beam energy of a few GeV
Tunable peak current > 1KA
Bunch rep. rate of the order of 1MHz
Inexpensive
Is it possible to replace
some of the SRF linac with a
DWFA linac??
Many hurdles to overcome as you will see…
6
FUNDAMENTALS:
Collinear
Dielectric Wakefield Acceleration
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Cylindrical Dielectric Wakefield Accelerator





Simple geometry
Capable of high gradients
Easy dipole mode damping
Tunable
Inexpensive
Recent results (obtained for Linear Collider development):
− 1000MV/m level in the THz domain (UCLA/SLAC group)
− 100 MV/m level in the MHz domain (AWA/ANL group)
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Wake field in dielectric tube induced by a short Gaussian
beam
e
Q
2b 2a
Cu
2

Q
1  2  z  
WZ ( z )  2 exp   
  cos(kz )
a
 2  n  
a=240 um; Q=1 nC; bunch length=0.5 ps (FWHM), f=650 GHz
Wakefield Amplitude Dependence on
Aperture or 1/f
300
100000
Ez(MV/m/10nC)
Wz(MV/m/1nC)
200
100
0
-100
-200
1000
100
10
1
0.01
-300
-0.25 0.25
10000
0.75
1.25
1.75
2.25
2.75
0.1
1
10
Inner Radius a (mm)
Distance (mm)
9
The Wakefield Theorem and the Transformer Ratio
Wakefield (MV/m/nC)
DRIVE
Collinear
Dielectric Wakefield
Acceleration
W+
W-


R=
WITNESS


The R< 2 limit has
kept interest in
collinear wakefield
accelerators to a
minimum.
W+= (Maximum energy gain behind the drive bunch)
W(Maximum energy loss inside the drive bunch)
<2
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Road map to a high energy gain acceleration
Methods to increase R>2
in a collinear wakefield accelerator
c
Ramped Bunch
 (z)
W+
z W
Reference: Bane et. al., IEEE Trans. Nucl. Sci. NS-32, 3524 (1985)
W
Ramped Bunch Train
(demonstrated at ANL)
z W
+
(z)
d
d
d
Reference: Schutt et. al., Nor Ambred, Armenia, (1989)
11
EXAMPLE:
A case study of an x-ray FEL user
facility based on a 2.4 GeV DWFA
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12
High rep. rate, X-ray FEL user facility
based on a 2.4 GeV DWFA
Quartz
DWFA
ID=400 um
freq = 850 GHz
FEL10
FEL2
FEL1
TR = 16.5
~30m
P=320 kW, 1 MHz
1.6 nC
13
Key technology:
DWFA RF structure design
Quartz DWFA
ID=400 um
ID, OD, Length
400 m, 464.7 m, 10 cm
e, tan
3.75, 0.6x10-4
Freq. of TM01, TM02, TM03
850 GHz, 3092 GHz, 5749 GHz
Q of TM01, TM02, TM03
1260, 3173,4401
r/Q of TM01, TM02, TM03
94.1 k/m, 3.2 k/m, 0.5 k/m
ng of TM01, TM02, TM03
0.592c, 0.794c, 0.813c
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How can a small DWFA can handle High Rep Rate????
RF packet ~333 ps
--cooling--
Quartz DWFA
ID=400 um
e
Collinear DWFA
• Ultra-short RF pulse (~333 ps)
• Heating is much less severe than
microwave accelerator
Average thermal heating
• Average power load 50 W/cm2
@100 kHz rep rate
RF pulsed heating
• DT ~ 20 ºC
15
Key technology:
drive bunch shaping enhances transformer ratio
Triangular bunch
TR~10
Double triangular
bunch TR~17
16
Key technology:
witness bunch generation

 E   W z  z dz
10 MeV in 10 cm

17
Double EEX technique:
a convenient tool for drive and witness bunch shaping
Emittance exchange
-I
Emittance exchange
FODO
T
-I
-I
-I
B
B
QD
QD
QF
QF
QD
QF
B
B
QD QF
QD QF
B
QD QF
QF
QD
B
B
QD QF
B
TM010 TM110 TM010
Deflecting cavity
x →z emit. exch.
z →x emit. exch.
mask
At EEX exit
(c)
2
1
0 -1
time (ps)
1200
1000
800
600
400
200
0
current (A)
After mask
witness
Before mask
-2
Drive and Witness from the same source bunch minimal timing jitter
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Key technology: How to handle beam loading:
Eacc=115 MV/m
Gaussian Electron bunch
• Large energy spread
• Strongly chirped in energy
Accelerated current
Wakefield
~DE=30 MV/m
19
Key Technology: Undulator



BAD: Accelerated beam is strongly chirped (little FEL gain)
BAD: Using the chirp to compress the beam does not seem to be useful for radiation
GOOD: For short beams (<10 m rms) the energy chirp is approximately linear in time
Strongly chirped beams for FEL applications
Longitudinal Gradient
Transverse Gradient
Tapering the undulator strength or
period can counteract large energy chirp
and maintain gain
Varying the undulator strength
transversely can counteract large energy
chirp and maintain gain
Δ𝛾
N
𝑡
Smaller undulator
strength K
Δ𝛾
𝛾0
Larger undulator
strength K
𝑥
S
20
Strongly chirped beams for FEL applications:
preliminary results
Example: Longitudinal Gradient
witness
beam
chirp
Tapering the undulator strength K
Power evolution of DWFA
beam + undulator taper
Linear
gain
Power profile near
saturation z/LG = 20
Chirped SASE spectrum
near saturation z/LG = 20
Nonlinear
regime
Some applications favor wide bandwidth
21
Gaussian witness bunch
110
Energy (MeV)
Can we reduce energy spread
due to beam loading?
100
90
80
70
15
10
5
0 5
z (um)
10 15
Gaussian
bunch
∙Q=50 pC
∙Edec=13.6 MV/m
∙Eacc=81.7 MV/m
∙sigmaE=5.3%
∙R=6
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Reverse triangular witness bunch
Drive-wake
110
Energy (MeV)
Key idea: Match the curvature of
the self-wake to the drive wake
100
90
80
70
20
10
0
10
z (um)
20
Reverse
triangular
bunch
∙Q=50 pC
∙Edec=6.3 MV/m
∙Eacc=86.3 MV/m
Witness self-wake
=0.3%
R=14
~20x reduction in energy spread
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Minimization of the energy spread in a witness bunch
Courtesy of E. Simakov, LANL
By additionally customizing the shape of the main bunch we
designed the configuration which minimizes the wakefield-induced
energy spread in the main bunch. The energy spread may be
made as low as 0.001%.
Beam pipe OD, 2b
1.14 mm
Dielectric tube OD, 2a
1.24 mm
Waveguide cutoff
298 GHz
Charge of the drive bunch
5 nC
Length of the drive bunch
2.127 ps
Charge of the witness bunch
250 pC
Length of the witness bunch
75 fs
Time between the bunches
9.4 ps
Transformer ratio
3.16
ΔG/G
1.5*10-5
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General (nonlinear) shapes are possible
Multi-leaf collimator:
• Used in medical linacs to shape the x-rays
• Each vertical leaf moves independently
Multi-leaf collimator
Varian's 120-leaf
multileaf collimator
leaf
Varian's high-definition
multileaf collimator
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Feedback on desired witness and drive shape
Emittance exchange
Multi-leaf mask
-I
-I
110
QD
QF
QD QF
B
B
QD QF
B
Energy (MeV)
B
QF
QD
Measured
Spectrum
100
90
80
70
20
10
0
10
z (um)
20
http://varian.mediaroom.com/i
ndex.php?s=31899&mode=gal
lery&cat=2473
FEEDBACK
26
BEGINNING EXPERIMENTAL STUDIES 1:
Demonstrate EEX based bunch
shaping at the Argonne Wakefield
Accelerator
27 27
Demonstrate bunch shaping using a double-dog leg
EEX beamline
at the AWA Facility
The
Argonne Wakefield Accelerator Facility
 Low Energy (14 MeV) beamline
RF
Photocathode
Gun
8 MeV
Linac
14 MeV
B2
B1
Quads
B1
B2
TDC
B3
B4
20
deg
Mask
Initial experimental goals:




Demonstrate bunch shaping and compare measured shape to 1st order theory
Measure EEX transfer matrix
Study 2nd order effects in beamline
Study space charge effects in beamline
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Demonstrate bunch shaping using a double-dog leg
EEX beamline
at the AWA Facility
The
Argonne Wakefield Accelerator Facility
 Low Energy (14 MeV) beamline
RF
Photocathode
Gun
8 MeV
Linac
14 MeV
Quads
B1
chirp
B2
B1
x’ slope
x, y beam size
B2
TDC
B3
B4
20
deg
multiple masks on
motorized actuator
Key tunable parameters
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Demonstrate bunch shaping using a double-dog leg
EEX beamline
Example: Experiment I - Shaping capability
Multiple masks will be used to study the bunch shaping capability of
the double dog-leg EEX beamline
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BEGINNING EXPERIMENTAL STUDIES 2:
Propagation of drive beam through a
10 meter DWFA linac at APS
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Drive bunch through a ID=400 m fiber !!!
Drive bunch:
•
•
•
•
ID=400 um
Charge = 1.6 nC
Normalized emittance = 2 m
Beam energy = 50 MeV (close to the accelerator end)
Beam size = 50 m (Beta function ≈ 10 cm)
Goal: Propagate drive bunch through meter scale DWFA
• With no focusing
• Beam size will triple in one meter!
• External focusing channel around dielectric
• ~10-20 cm focal length
• Control SBBU with BNS damping
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10 m long structure test in APS LEUTL tunnel
1. APS will install LCLS type e-gun in 2013
• 0.5 nC, 500 fs, 1 m bunches
• Beam into the LEUTL tunnel in 2014
2. Propagate beam through 10 m long DWFA at APS
•
•
•
•
Single Bunch Beam Break Up (SBBU)
Vacuum pumping
Cooling design
etc.
LEUTL tunnel is ~ 40 m long and is
ready to accept the beam
Some equipment exists, new
equipment and diagnostics will be
needed
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Summary
 The concept: High Repetition-Rate, Soft X-ray FEL User Facility
– 10 DWFAs linacs driven by a single SRF linac
– 10 FEL lines @ 100 kHz rep. rate.
– Compact, Inexpensive, and Flexible
 A working group has started feasibility studies
–
–
–
–
–
Parameter studies of the overall concept
Bunch shaping studies at the AWA facility
Beam propagation through a 10m DWFA linac at APS
Modeling of the large energy spread in the FEL
Many more:
• Drive and witness jitter
• Dielectric breakdown limitation testing
• Etc.
 We welcome collaborators and new ideas!
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